Application information L9700D-E
10/14 Doc ID 022301 Rev 1
The C
IN
capacitors must be used only on analog inputs because they present a low
impedance during the sampling period.
The minimum value for C
IN
is determined by the accuracy required, the time taken to
sample the input and the input impedance during that time, while the maximum value is
determined by the required frequency response and the value of R
S
.
Thus for a resistive input A/D connector where:
T
S
= sample time (seconds)
R
D
= device input resistance (Ohms)
V
IN
= input voltage (Volts)
k = required accuracy (%)
Q
1
= charge on capacitor before sampling
Q
2
= charge on capacitor after sampling
I
D
= device input current (Amps)
Thus:
but Q
1
= C
IN
V
IN
and Q
1
– Q
2
= I
D
– T
S
so that
and
so
The calculation for a sample and hold type convertor is even simpler:
k = required accuracy (%)
C
H
= hold capacitor (farad)
Q
1
Q
2
–
kQ
1
⋅
100
-------------- -=
I
D
T
S
kC
IN
V
IN
–⋅
100
-------------------------------- -=
C
IN
min()
I
D
T
S
⋅
V
IN
k⋅
-----------------
Farad=
C
IN
min()
100 T
S
⋅
kR
D
⋅
---------------------
Farad=
C
IN
min()
100 C
H
⋅
k
----------------------
Farad=
